A pressure gauge calibration apparatus in the related art has been known which uses a dead weight pressure balance as a standard device. The apparatus compares a pressure applied to a calibrated device by the dead weight pressure balance with an indicated value of the calibrated device (see FIG. 1). The dead weight pressure balance is excellent in long-term stability and a generated pressure thereof is hardly affected by a calibration procedure. Accordingly, it is possible to calibrate a pressure gauge in any desired calibration procedure. It is also possible to evaluate, for example, a hysteresis characteristic and a time-dependent change characteristic of the pressure gauge that is the calibrated device.
Calibration using a calibrated pressure gauge as the standard device without using the dead weight pressure balance also has been known in the related art (see FIG. 2). The same pressure is always applied to the pressure gauge that serves as the standard device and the calibrated device during the calibration. The calibrated device is calibrated by comparing two outputs at each calibration pressure point. This method relatively facilitates automation, thereby enabling an efficient calibration.
The pressure gauge serving as the standard device is calibrated by a superior standard device in advance. A calibration value thereof is however affected by the calibration procedure. FIG. 3 illustrates that in a case of a general step-wise calibration procedure, the calibration values are different from each other during a pressure rising process and a pressure dropping process due to a history effect. FIG. 3(a) illustrates a time change in a pressure received by the pressure gauge in the general step-wise calibration procedure. At each calibration pressure point, measurement is performed when certain waiting time passes after pressure adjustment. The time for measurement is illustrated by a black circle in the drawing. FIG. 3(b) illustrates a calibration result obtained by the step-wise calibration procedure. A horizontal axis represents the pressure, and a measurement result on a vertical axis represents a relative deviation of an output in the pressure gauge from a standard value. A significant difference (referred to as a hysteresis difference) can be seen in the measurement results between the pressure rising process and the pressure dropping process.
FIG. 4 illustrates that the result of the pressure dropping process is affected when the maximum pressure of the calibration in the step-wise calibration procedure is changed. FIG. 4(a) illustrates the time change in the pressure received by the pressure gauge. The maximum pressure is changed into three types without changing the time interval for the measurement. In FIG. 4(b), the results obtained with the three type calibration procedures are compared with one another. The horizontal axis represents the pressure, and the vertical axis represents the relative deviation of the output in the pressure gauge from the standard value. The measurement result during the pressure dropping process is greatly affected by the maximum pressure in the calibration. As the maximum pressure is lowered, the hysteresis difference is decreased.
The present inventors have previously proposed the following finding in Non-Patent Document 1. In order to be less sensitive to the calibration procedure as described above, a pressure is necessarily restored to a pressure zero state (for example, in a case of a gauge pressure, an atmospheric pressure release state or a standard atmospheric pressure state may be acceptable) after data acquisition at each calibration pressure point as illustrated in FIG. 5. In this procedure (referred to as a 0-A-0 type calibration procedure), the hysteresis difference is decreased and the measurement result is less affected even when the maximum pressure in the calibration is changed as illustrated in FIG. 6.
FIG. 5(a) illustrates the time change in the pressure received by the pressure gauge in the 0-A-0 type calibration procedure. FIG. 5(b) illustrates the calibration result obtained during the 0-A-0 type calibration procedure. The horizontal axis represents the pressure, and the vertical axis represents the relative deviation of the output in the pressure gauge from the standard value. As compared to the step-wise calibration, the difference is much decreased between the pressure rising process and the pressure dropping process (see FIG. 3(b)). FIG. 6(a) illustrates the time change in the pressure applied to the pressure gauge. The maximum pressure is changed into three types without changing the time interval for the measurement. In FIG. 6(b), the results obtained during the three type calibration procedures are compared with one another. If the 0-A-0 type calibration procedure is adopted, the influence of the maximum pressure on the calibration result can be suppressed to be much smaller.